Anthrax is an acute disease caused by the gram-positive bacterium Bacillus anthracis. Among the primary cellular targets are mononuclear phagocytes, which undergo both apoptosis and necrosis as a result of exposure to toxins produced by Bacillus anthracis. Lethal toxin (LT) is composed of two protein subunits, Protective antigen (PA) and Lethal Factor (LF). PA is a membrane-integrating protein that facilitates LF entry into cells. LF is a metalloprotease that specifically cleaves the NH2 termini of MAP kinase kinases (MAPKKs). The consequential inhibition of the activation of MAP kinases is thought to be central to Bacillus anthracis' evasion of the innate immune response. We present evidence that treatment of macrophages with interferon beta (IFNbeta) or interferon gamma (IFNgamma) promotes survival of macrophages after LT exposure. Although IFNbeta or IFNgamma does not protect macrophages from LT induced cleavage of MAPKKs, it does protect cells from cleavage of two important protein tyrosine phosphatases (SHP-1 and SHP-2), that have not been previously identified as substrates for LT. These findings suggest that interferons may provide a potential therapeutic approach to inhibit the biological actions of LT through prevention of the proteolysis of newly identified substrates that are targets of LT. We propose to: Aim 1: Identify proteins other than MAPKKs that are substrates for LT and determine which of these substrates are protected from LT mediated proteolysis in macrophages incubated with IFNbeta or IFNgamma. Aim 2: Determine the sites of cleavage of SHP-1 and SHP-2 as well as other proteins that are both proteolyzed by LT and are protected from proteolysis in cells exposed to LT and IFNgamma or IFbeta.